Lightning arrester with main and preionizing gaps



Jan. 13, 1970 I T. J. CARPENTER 3,489,949

LIGHTNING ARRESTER WITH MAIN AND PREIONIZING GAPS Filed Nov. 13, 1967United States Patent 3,489,949 LIGHTNING ARRESTER WITH MAIN ANDPREIONIZING GAPS Thomas J. Carpenter, Pittsfield, Mass, assignor toGeneral Electric Company, a corporation of New York Filed Nov. 13, 1967,Ser. No. 681,991 Int. Cl. H01j 7/44, 13/46' U.S. Cl. 315-36 14 ClaimsABSTRACT. OF THE DISCLOSURE A trigger gap is connected in shunt circuitrelation to at least one of at least two series connected main gapswhose voltage distribution under increased applied voltage is upset by ashunt grading impedance network. The trigger gap is externallyadjustable and preionized and the grading network comprises linear andnonlinear resistors. Coupling and decoupling impedances are connectedbetween the trigger gap and the rest of the circuit.

3,489,949 Patented Jan. 13, 1970 improved means for producing cascadedsparkover of a lightning arrester multigap unit.

A further object of the invention is to provide a multigap unit whose 60cycle sparkover voltage can be adjusted to narrow tolerances after gapassembly.

An additional object is to provide a multigap unit whose main gapspacings are not critical.

Still another object is to provide a multigap unit having a constantsparkover, that is, the sparkover is not affected by the main gapelectrode surface conditions.

An added object is to provide a multigap unit which will interrupt withan applied 60 cycle voltage in the order of 90% of its sparkovervoltage.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

In the drawing,

FIG. 1 is a combination schematic representation and circuit diagram ofa preferred embodiment of the in- I vention, and

FIG. 2 is a partially disassembled perspective view of the embodiment ofthe invention shown schematically and diagrammatically in FIG. 1.

The multigap unit comprises gaps 1, 2, 3, 4, and 5 in L the form of horngaps connected in series between termiceed the insulation strength ofthe connected apparatus.

After the surge has passed, the follow current produced by the normalcircuit voltage is limited by the resistor to a value which can beinterrupted by the gap.

Modern lightning arresters for high voltage circuits have asophisticated multigap unit in the form of a plurality of gaps in seriesprovided with means for producing cascaded sparkover. This brings thesparkover voltage of the gap unit as a whole closer to its interruptingvoltage which is generally desirable.

Heretofore there have been two means for producing cascade sparkoverboth of which require an impedance networkshunting the gaps for gradingthe total voltage distribution among them prior to sparkover of any oneof them. One means has comprised a linear impedance grading network anda fixed trigger gap shunting one of the main gaps and having a lowersparkover voltage than the main gaps. The other means has comprised anonlinear impedance grading network for upsetting the voltagedistribution in response to increased voltages. The latter requiresaccurate gap spacing and small tolerances in the impedance values of thenetwork.

In accordance with this invention, there is provided a new and improvedmultigap unit whose sparkover voltage can be adjusted to narrowtolerances after assembly, whose main gap spacings are not as critical,whose sparkover voltage is independent of main gap electrode conditions,and which will interrupt with an applied voltage of the order of 90% ofits sparkover voltage.

Briefly, the invention comprises a preionized externally adjustabletrigger gap shunting one of a plurality of series connected main gapswhich in turn are shunted by a voltage grading network preferablycomprising nonlinear resistors. In addition, one or more coupling anddecoupling impedances may be connected between the trigger gap and therest of the circuit.

An object of the invention is to provide a new and improved lightningarrester multigap.

Another object of the invention is to provide new and nals 6 and 7.Actually these gaps are not in the same plane as shown in FIG. 1 but arein parallel planes in stacked or superposed relation as shown in FIG. 2wherein they are sandwiched between insulating plates or discs 8 havingregistering edges and forming therebetween arcing chambers. The gap 5 isshunted by a magnet coil 9 for producing a magnetic flux normal to thegap spacings and of such direction as to propel arcs in the gaps 15 outto the extremities of their horn electrodes, i.e., the coil 9 reinforcesthe arc propelling action of the horn gap electrodes themselves.

Shunting the gaps is a gap voltage controlling or grading networkcomprising linear resistors 10 and 11 in series with nonlinear valvetype resistors 12 and 13. As shown, there are intermediate crossconnections 14, 15, 16 and 17 for connecting the linear resistors 10 and11 respectively in shunt circuit relation with the gaps 1 and 3 and forconnecting the nonlinear resistors 12 and 13 respectively in shuntcircuit relation with the gaps 2 and 4.

Under normal conditions, practically the entire applied voltage betweenthe terminals 6 and 7 will be distributed substantially equally acrossthe gaps 1, 2, 3, 4 inasmuch as under these conditions the inductance ofthe coil 9 is negligible and its resistance predominates so that theseries circuit of the grading resistances is completed through the coil9. However, under impulse voltage conditions, the coil 9 has a highinductive reactance whose ohmic value materially exceeds its ohmicresistance under these conditions. A by-pass capacitor-.18 is thereforeconnected in shunt circuit relation with the coil 9 so that underimpulse conditions th voltage distribution among the gaps 15 is upset inthat the reactance of the capacitor 18 becomes very small and theresistance of resistors 12 and 13 greatly decreases with the result thatthe voltage across the gaps 1 and 3 becomes substantially higher thanthe voltage across the gaps 2, 4, and 5.

Gaps 1 and 3 are provided with resistance ballasted preionizer gaps inthe form of resistors 19 and 20 respectively in series with gaps 21 and22. Likewise gaps 2 and 4 are provided with preionizers 23 and 24 in theform of a dielectric tube 25 having an inner cylindrical electrode 26and an outer cylindrical electrode 27. There is minimum overlap betweenthe inner and outer elec- 3 trodes so as to produce minimum capacitancebut due to the termination of 27 on the surface of the dielectric tube25 it produces high stress concentrations when gaps 1 and 3 sparkovercopious corona indicated at 28 for irradiating and preionizing theirassociated main gaps. The principal purpose of the preionizers is toreduce the time lag and increase the consistency of the sparkover oftheir associated main gaps. The resistance ballasted preionizer gaps 21and 22 are better for slower long time voltage surges and thepreionizers 23 are for the transient conditions resulting from sparkoverof gaps 1 and 3.

The gap 1 is provided with a trigger gap 29 located between the gap 1and the preionizer gap 21. As shown in FIG. 2, one of its electrodes isa screw 30 accessible through a lateral space in the stack of discs 8 sothat the spacing of the trigger gap can be adjusted externally.

A decoupling resistor 31 is connected between the main gap 1 and thetrigger gap 29 for decoupling these elements so that sparkover of thetrigger gap 29 will not keep the voltage of main gap 1 too low for it tosparkover. Also a coupling capacitor 32 is connected in parallel withthe resistor 31 for lowering the effective impedance of resistor 31 whenthe trigger gap 29 sparks over. Capacitor 31 also tends to compensatefor the stray capacity of main gap 2 during this transient period.

The operation of the invention is as follows. As the total impressedvoltage across the multigap between its terminals 6 and 7 increases, thepreionizer gaps 21 and 22 first spark over irradiating trigger gap 29and main gap 3 and at the same time the preionizers 23 and 24 alsoirradiate their main gaps and prepare them for fast consistent sparkoverwhen the voltage attains a somewhat higher value. The ballast resistors19 and 20 of course limit the current through the preionizer gaps 20 and21 to a very low value. The trigger gap 29 being closer to thepreionizer gap 21 than the main gap 1 is more extensively preionizedthan main gap 1.

At the same time, the resistance of the nonlinear grading resistors 12and 13 decreases due to their inherent characteristic so that the normalsubstantially equal voltage distribution across the gaps 1-4 is alteredand the voltage across gaps 1 and 3 becomes considerably higher than thevoltage across gaps 2 and 4. At a voltage somewhere between thesparkover voltage of the gap 1 and half that value, the trigger gap 29sparks over irradiating main gap 1, short circuiting the linear resistor11 and practically instantaneously raising the voltage of the main gap 3to well above its sparkover voltage. Main gap 3 then sparks over. Thisgreatly reduces its voltage so that a still greater proportion of thetotal applied voltage is rapidly impressed across gaps 2 and 4 whichwill sparkover next. This then further increases the voltage across gaps1, and and 1 will sparkover next followed finally by the sparkover ofcoil shunting gap 5. While trigger gap 29 is sparked over it acts as apreionizer for main gap 1. It should be understood, of course, that allof these cascading actions take place practically instantaneously. Aftersparkover of main gap 1, the trigger gap 2 will clear as its voltagewill then be substantially zero for a time.

From the above description, it will be seen that the adjustment orspacing of the main gaps is not critical and consequently that spacingcan be purposely increased substantially beyond what it would have to beif there were no trigger gap and the sparkover of the assembly was to bethe same. The adjustability of the trigger gap 29 permits accurateadjustment of the whole assemblys sparkover even when the impedancecomponents have greater variations than would be allowable with acircuit not having a trigger gap. Furthermore, the 60 cycle sparkovercan be set accurately over a relatively wide range which remainsunchanged by repeated discharges due to the fact that the trigger gap 29only carries limited current for a brief instant. As a result, testshave shown that the assembly can interrupt an applied voltage l in theorder of of its sparkover voltage. The discharge current of the triggergap 29 is limited by the series impedances 10, 12, 13 and 18. Thesefactors result in the electrode surface of the trigger gap 29 beingrelatively unaifected by the applied discharge current passing throughthe main gaps. The air density in the trigger gap 29 is relativelyconstant as the discharge current of the trigger gap 29 is low inmagnitude and duration and the dielectric strength of the trigger gap 29recovers during the passage of current in the main gap 1 and canwithstand approximately sparkover voltage when the main gaps interruptthe current. The effect of current discharge on the main gap sparkoveris in turn minimized by the increased gap spacing that is permitted bythe use of the trigger gap 29.

In a specific multigap assembly in accordance with this invention whichis rated 6 kv., the main gap spacing is 50 milsi3 mils. The preionizers23 and 24 have approximately 9 picofarads. The preionizers 21 and 22have gaps of approximately 24 mils and the ballast resistors 19 and 20have resistance of about 6.8 megohms. The nonlinear resistors 12 and 13have a voltage drop of 1650 voltsiSO volts when passing a current of .2milliampere DC. The linear resistors 10 and 12 have about 12.5 megohmsresistance. The trigger gap 29 is adjustable between 26 mils and 35mils. The resistor 31 has a resistance of 5000 to 20,000 ohms. Thecapacitor 18 has a capacitance of about l550 picofarads.

While there have been shown and described particular embodiments of theinvention, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention, andtherefore it is intended by the appended claims to cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a multi-gap unit for a lightning arrester, at least two generallysimilar serially connected main gaps, an impedance network forcontrolling the voltage distribution of said main gaps before theirsparkover comprising at least two separate impedances connectedrespectively in shunt circuit relation with said main gaps, a triggergap connected in shunt circuit relation with one of said main gaps andphysically adjacent thereto, said main gaps having a sparkover voltageof between one and two times the sparkover voltage of said trigger gap,and a preionizer connected in shunt circuit relation with said triggergap and physically adjacent thereto.

2. A gap unit as in claim 1 in which said trigger gap is physicallybetween its main gap and its preionizer.

3. A gap unit as in claim 1 including means for adjusting the spacing ofsaid trigger gap.

4. A gap unit as in claim 3 in which said gaps are sandwiched in a stackof insulating separator discs and said means for adjusting the spacingof the trigger gaps is accurate from outside said stack.

5. A gap unit as in claim 1 including a second preionizer connected inshunt circuit relation with and physically adjacent the other main gap.

6. A gap unit as in claim 1 in which said impedances are respectivelylinear and nonlinear.

7. A gap unit as in claim 6 in which the impedances are resistors andthe nonlinear resistor has a characteristic generally similar to aseries valve resistor for a lightning arrester, the linear resistorbeing connected in shunt circuit relation with the main gap having thepreionized trigger gap.

8. A gap unit a in claim 1 in which said preionizer is a series resistorballasted gap having a lower sparkover voltage than said trigger gap.

9. A gap unit as in claim 5 in which said second preionizer is a highlyelectrically stressed dielectric type.

10. A gap unit as in claim 5 in which said second preionizer is a gaphaving a higher sparkover voltage than the trigger gap.

5 6 11. A gap unit as in claim 1 having a decoupling re- 3,356,89412/1967 Lafferty 315-36 X sistor in the shunting connection between saidtrigger 3,377,503 4/1968 Osterhout 317-70 X gap and its associated maingap. 3,414,759 12/1968 Connell et al 31770 X 12. A gap unit as in claim10 having a decoupling 3,418,510 12/1968 Melhart 315-36 X resistor inthe shunting connection between said second preionizer and said othermain ga 5 JAMES W. LAWRENCE, Primary Examiner 13. A gap unit as in claim11 having a coupling capaci- C R CAMPBELL Assistant Examiner tor inshunt with said decoupling resistor.

14. A gap unit as in claim 12 having a coupling ca- X- pacitor in shuntwith said decoupling resistor. 10 70 References Cited UNITED STATESPATENTS 3,328,632 6/1967 Robinson 31536X 3,348,100 10/1967 Kresge 3177O

